The present invention relates generally to serial printing devices in which a type member is moved along a line in the printing direction, and more particularly, to a small printing device in which a conversion mechanism providing first and second outputs alternatively is provided. The first and second outputs are alternately switched to select type elements and for printing. Such a small printing device is applicable to a portable or pocket-size electronic computer.
Examples of a conventional printing devices in which selection of type elements used for printing is controlled using a differential mechanism as a conversion mechanism capable of providing first and second outputs are disclosed in U.S. Pat. Nos. 3,548,993 and 4,043,439. Disadvantageously in each of the conventional printing devices, the carriage is intricate in configuration and bulky because the type member, the printing mechanism and the selection mechanism are mounted on the carriage.
In general, a conventional printing device combined with a portable electronic computer prints symbols at the two lowest print positions and numerals at the remaining lateral positions of a line. In the printing device disclosed in U.S. Pat. No. 4,043,439, symbol types and numeral types are arranged on the outer wall of a single type wheel. Accordingly, the diameter of the type wheel is necessarily large and hence, it is impossible to miniaturize the printing device. In the case where symbols are printed at two lowest positions and numerals are printed at the remaining positions, the desired type element must be selected from among all of the elements available, which takes a relatively long time and makes the printing speed slow. Furthermore, the conventional device suffers a drawback in that, if it is intended to increase the number of type elements available for printing, the type head necessarily becomes larger.
Presently, there is strong demand to reduce the size of pocket-size electronic computers with a printer. However, it is considerably difficult to miniaturize a pocke-size electronic computer using the above-described conventional printing device in which the carriage is large and intricate.
In the conventional printing device disclosed in U.S. Pat. No. 3,548,993, electromagnets are provided for the first and second outputs of the differential mechanism. Therefore, the printing device is large in size and high in manufacturing cost. Furthermore, since current is continuously applied to the electromagnets during the printing operation, power consumption is relatively large.
This drawback has been eliminatted by the printing device disclosed in U.S. Pat. No. 4,043,439 to some extent. In that printing device, only one electromagnet is used, and the application of current to the electromagnet is suspended relatively early in the printing operation. However, in addition to a switching mechanism for switching type selecting operation and a printing operation, a positioning element for holding a selected type element during the printing operation must be employed by the printing device. Therefore, necessarily the number of components forming the printing device is large and the printing device is intricate in construction and large is size. Neither of the aforementioned patents provides a detailed description of place shifting, sheet feeding and carriage returning operations. Each of these operations needs a corresponding mechanisms and power source. Accordingly, the conventional printing devices are not suitable for pocket-size electronic computers due to their intricate construction, bulky size and high manufacturing cost.
An example of a conventional serial printing device for a small portable electronic computer has been disclosed in U.S. Pat. No. 4,051,942. This conventional serial printing device also has an intricate construction and large electromagnetic plungers are used for printing, place shifting, sheet feeding and carriage returning operations. Furthermore, the device uses a pulse motor to select type elements. Accordingly, the device is large in size and high in manufacturing cost. In addition, the device uses wire to shift the printing location. Since the wire is liable to stretch or move sidewardly, the printing position may not be accurate.
Another conventional printing device has been disclosed by U.S. Pat. No. 3,876,053 in which a rack and a shifting cam are used as a shifting mechanism. In that printing device, the type selecting mechanism is considerably intricate, and the printing mechanism is so designed that the type head is locked by a tab mounted between the frames. Therefore, that printing device is also intricate in construction and large in size.
What is needed is a printing device which is of simple construction, has few parts, small size and consumes little electrical energy.